Waste incineration is indispensable to industry for reducing the weight of waste and eliminating the danger of fire, pathogenic organs, and toxic organic compounds. In addition to harmful or toxic compounds (pollutants), which exist in waste and remain after combustion, such as arsenic, heavy metals and radioactive substances, pollutants are also produced by incineration even if waste has no pollutants. Pollutants produced in incineration are causing problem today. Rotating reactor is adopted and/or lime is added in incineration, however, effective results cannot be obtained.
Burning waste needs oxygen (O.sub.2). Only 1/5 of the air being O.sub.2, burning one metric ton dried waste needs fifteen ton (11,000 m.sup.3) air. Major thermal substances of the incineration are water (H.sub.2 O) air and exhaust. Exhaust has usually high temperature such as 600-1500 degrees C. Heat of vaporization of H.sub.2 O (latent heat) and heat which is used for heating the exhaust (sensible heat) are wasted into the atmosphere. The latent heat is a large amount of energy. Therefore the total combustion heat (higher heat value) is distinguished from that omitted the latent heat of (H.sub.2 O) (lower heat value). Living organisms use higher heat value, but common burning uses only lower heat value. While (H.sub.2 O) is not only produced during burning, but is also found in waste, latent heat of H.sub.2 O from both sources is wasted. Therefore, 2-5 times of weight of waste auxiliary fuel is consumed in raw garbage incineration. Incinerating waste including 80-90% H.sub.2 O such as sewerage dregs cake, filter remained tofu scum (like a brewer's grains of tofu making) and wasted tofu bean-curd, even 10 times of weight auxiliary fuel can only produce carbides including raw part. In general in these cases, extraordinary amount of auxiliary fuel is consumed in waste incineration.
Heat recycle arts are described.
Let air and exhaust have the same heat capacity and flow in sufficiently long pipes, the exhaust being hot and air being cold at each intake. When the two pipes are contacted along their whole length with the intakes being adjacent and the outlets being adjacent, the exhaust and air at the outlets have the same average temperature. This arrangement is called a parallel flow heat exchange (PF). On the other hand, when the pipes are so contacted that each intake is adjacent to the other outlet, then the temperature at each outlet becomes the same as the temperature at the corresponding or adjacent intake. This arrangement is called a counter flow heat exchange (CF). CF method has been tried in the dilute fuel combustion.
When a higher temperature air is used, the lower concentration flammables can be ignited, and nitrogen oxides (NO.sub.x), carbon monoxide (CO), soot, etc. can be reduced, and efficiency of energy transformation rate is improved. In some cases trials were conducted wherein fuel combustion and H.sub.2 O vapor in exhaust was not condensed into liquid.
Waste incineration is much more difficult than proper fuel combustion, because waste includes a large amount of H.sub.2 O, many nonvolatiles, solids or various substances, ash remains or residuals. Burning waste produces pollutants easily and has various shapes. Therefore, it has been considered impossible in waste incineration to use high technologies, which are difficult to use even in fuel combustion.
Pollutants can be classified into two groups: inevitable and additional. Inevitable pollutants are pollutants which cannot be avoided and include carbon dioxide, CO.sub.2 and nitrogen oxides NO.sub.x. If the main flammable components of waste are generally organic compounds, waste incineration produces CO.sub.2 and H.sub.2 O. CO.sub.2 is not very harmful, but a person can die if he is exposed to it in high concentration. Since CO.sub.2 is a global greenhouse effect substance, the reduction of its emission is required. Auxiliary fuel also produces CO.sub.2. The reduction of auxiliary fuel is necessary for saving natural resources, too. CO.sub.2 produced by auxiliary fuel should be reduced much more than CO.sub.2 produced by waste itself. Incineration of waste formed of a mixture of plastics is, however, is a problem because it produces pollutants such as dioxins. Electric generation exploiting the heat from waste incineration is recommended, however, saving auxiliary fuel is more effective for present technology.
NO.sub.x is produced only when the air is heated and its formation enthalpy is positive so that it is decomposed by catalysts. A catalytic converter of a car utilizes this effect. NO.sub.x is hardly produced in incomplete combustion and is reduced when complete combustion gas (oxidizing flame) is mixed with incomplete combustion gas (reducing flame).
Additional pollutants are classified further into nonflammable and flammable pollutants. Nonflammable pollutants include halogen, hydrogen halides, NO.sub.x, sulfur oxides (SO.sub.x), phosphorus oxide (PO.sub.x) and fly ash. Flammable pollutants include organic compounds, carbon, CO, ammonia (NH.sub.3), hydrogen cyanide (HCN), hydrogen sulfide (H.sub.2 S) and sulfur. Organic compounds include organic halogen compounds, amines, nitrites, mercaptans, hydrocarbons, alcohol, aldehydes, organic acid and soot. During a complete combustion flammable pollutants changes into inevitable pollutants and/or nonflammable pollutants. However, flammable pollutants such as organic halogen compounds cannot be easily combusted completely. The pharmacopoeia of many countries and international organizations (USA, Japan, UK, France, European Pharmacopoeia, International Pharmacopoeia, etc.) mentions the "oxygen flask combustion method" to measure the quantity of and/or identify halogen (Br, Cl, F, I) or sulfur (S) included in organic compounds. The title of the mentioned pharmacopoeia of USA is "Oxygen Flask Combustion". This method comprises the following steps. The organic substance is set in a filter paper in a platinum (Pt) basket, it is burned in pure O.sub.2 atmosphere and the quantity of acid gas such as hydrogen chloride (HCl) produced in combustion is measured. Even substance which are very difficult to burn, such as organic halogen compounds can be completely burned under certain favorable condition, such is pure O.sub.2 and in the presence of a Pt catalyst.
If additional air is applied in order to achieve complete combustion, then additional auxiliary fuel is also required to provide the sensible heat. From an economical point of view, an incomplete but nearly complete combustion is preferable.
Dioxins are a kind of organic halogen compound, which is an organic compound combined with chlorine when waste is scorched, and is produced by the burning of organic halogen compounds such as vinyl chloride. Halogen elements and halogen compounds, which vaporize in an incinerator, are called volatile halogens. Organic halogen compounds decompose thermally above 180 degrees C. and result in volatle halogens such as HCl or Cl.sub.2. Aluminum chloride AlCl.sub.3 (b.p. 183 degrees C.) is also a volatile halogen. Volatile halogens change organic compounds into organic halogen compounds. Production of dioxins can be observed even in the incineration of waste, which includes no organic halogen compounds, when volatile halogens are produced. Carbon reacts with hydrogen to produce organic compounds at high temperatures.
Even if organic halogen compounds are decomposed thermally, decomposed materials reform into organic halogen compounds again. Volatile substances condense in the cooler neighborhood than the boiling point zone. Organic halogen compounds are volatile at the fire room temperature. Organic halogen compounds cannot be easily vaporized in waste clusters, blocks or wastes buried in ash, and remain or condense therein, because the raw parts of waste have low temperature. Thereafter, organic halogen compounds may be found in ash when raw parts of waste burn out and the ash cools off for lack of flammables. This is the mechanism, which results in dioxin being found in the burned out ash.
NO.sub.x are produced only in the presence of air. More NO.sub.x is produced during burning of waste which includes nitrogen element (N). The waste, which produces acid gas in complete combustion, includes specific elements (N, F, Cl, Br, I, S, P, etc.), which are called acid pollutant elements. Waste including N produces NO.sub.x in complete combustion and produces a flammable pollutant during incomplete combustion which may be referred to as an acid pollutant such as amine, ammonia, and hydrogen cyanide) in incomplete combustion. Acid pollutant is an acid gas, therefore it can be removed by alkaline aqueous solution in the scrubber. But it is impossible that have the scrubber connected to the machine directly or integrated with the machine because the high temperature exhaust makes the scrubber solution boil.
Combustion is a chemical reaction and raw materials react to produce reaction products (positive reaction). At the same time, the reaction products react to produce raw materials (negative reaction). Both reactions are accelerated at high temperature. Negative reaction can hardly happen if the reaction products are removed. Removal of the reaction products to restrain the negative reaction is referred to as disproportionation. While gases react very fast, liquids slower, solids very slow. For example, fuel gas burns explosively but charcoal and coke burn for a long time. If a part of the reaction products become solid (which do not vaporize, melt or decompose) then the negative reaction can hardly happen as well. This is also a type of disproportionation. While it is difficult to reduce the negative reaction to 1/10 by reaction temperature variation, it is easy to make it less than 1/1,000 using disproportionation.
We now consider the materials (basic substances) that react with acid pollutant easily to yield harmless solid compounds (salts) which do not vaporize, melt or decompose even at high fire room temperatures. Basic substances render acid pollutants harmless. Metals, oxides, hydroxides, carbonates, hydrogen compounds, organic acid salts, alcohlates and organic metal compounds, which include alkali metals (K, Na, etc.) or alkaline earth metals (Ca, Mg, etc.), are examples of basic substances. The disproportionation of acid pollutants using basic substances is called neutralizing fixation. The total valence number of a base element per acid pollutant element in the reaction is called a stoichiometric ratio. The valence of alkali metals is one and the valence of the alkaline earth metals is two. The stoichiometric ratios of N, halogen, S and P are one, one, two and three, respectively. The stoichiometric ratio of 1 for N is only practical under 300 degrees C., because its salts decompose above that temperature. Halogen salts such as KCl and CaCl.sub.2 melt under the combustion temperature so that the stoichiometric ratio of halogens is practically 1 only under that temperature. However halogen oxides make their melting points higher, the stoichiometric ratio of halogens can practically be `two`.